(57) 57 A bridge network 1 of current switching devices 2, 3, 4, 5 is operated in full
wave manner by controlling signals A, B, C, D respectively produced by two similar,
parallel and pulse width modulated power control circuits 12,13 connected to receive
a common input signal, one pair of opposing switch devices (2, 5) being controlled
by one circuit 12 and the other pair 3, 4 being controlled by the other circuit 13.
The pulse width control inputs 14 of the circuits 12, 13 are connected to voltage
sensing means in the bridge supply circuit so that the pulse width of the control
signals A, B, C, D is determined in accordance with the bridge supply level. The arrangement
is such that the control circuits 12, 13 although not be designed to fail-to-safety
standards operate in a fail-safe manner to provide a constant level output of the
input signal.
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